This invention relates to spray forming of porous metals at high deposition rates and particularly relates to porous metals having spherical pores. 2. Review of the Prior Art
In high deposition rate spray forming, a stream of molten metal is typically atomized by an inert gas, producing a spray of droplets that are accelerated towards the substrate. The spray impacts the substrate and consolidates upon it to form a nearly fully dense deposit, termed a preform. The metal flow rate, superheat, flight distance, and atomization gas pressure are controlled so that the correct ratio of liquid to solid material is delivered to the preform surface. Liquid permits incoming droplets to be fully incorporated into the preform without boundaries between successive splats. Fracture during impaction breaks up the dendritic structure of incoming particles, and coarsening during cooling generates an equiaxed structure with the scale of segregation limited to tens of microns.
Spray forming offers considerable microstructural refinement, as does conventional powder metallurgy (P/M) processing, and additionally eliminates many of P/M's powder handling and compaction stages such as sieving, storage, cold pressing, and sintering. It has been successfully applied to a wide range of alloys and metal matrix composites. Spray forming can also produce fully dense preforms that can be roll extruded into IN625 piping with mechanical properties equivalent to conventionally processed material at cost savings as high as 30-50% compared to conventional ingot metallurgy.
Currently there are a number of spray forming pilot plants producing rolls, thin strip, and extrusion billets in copper, aluminum, and steel alloys. U. S. Pat. No. 5,110,631, for example, teaches the production of metal or metal alloy spray deposits using an oscillating spray for continuous length or for producing tubular, roll, ring, cone, or other axi-symmetric shaped deposits of discrete length, a controlled amount of heat being extracted from the molten metal or metal alloy in flight and/or on deposition and base porosity being considerably reduced with continuous production techniques involving a single pass.
In spray forming of conventional engineering materials, considerable effort has been directed at elimination of porosity in the preform. Such porosity can be generated by a number of different mechanisms. One of the most prevalent is porosity caused by lack of sufficient liquid in the spray to fill interstices and completely weld solid particles delivered to the preform surface. `Cold` porosity can also be caused by excess heat removal from the preform, thereby creating a solidified surface. Often these conditions are present at the edge of the spray and in the first few millimeters deposited on an unheated substrate. When depositing material in multiple passes, a banded structure of dense material layered with porosity can be formed. Other types of porosity are associated with particulate injection, rejection of dissolved gas during solidification, or excessive splashing/turbulence on the preform surface. In many cases, these problems are eliminated or minimized through proper spray conditions and substrate selection. Post processing such as hot rolling, extrusion, and hot isostatic pressing (HIPping) has been effectively used to achieve full density and mechanical properties superior to wrought ingot metallurgy product.
In many materials, a limited amount of porosity is accepted, and its effect on mechanical properties is allowed for in the design process. In P/M materials, porosity is an artifact of the consolidation process and is frequently accepted in a trade-off for increased control of distortion and reduction of sintering time or temperature. An open cell pore geometry can be produced by incomplete sintering and is utilized to contain oils in self-lubricating bearings, as flame arresting inserts, as metallic filters, and in a variety of other applications.
Fewer applications have been found that take advantage of the better mechanical properties of closed cell metallic materials. One of the reasons for these better properties is that angular, interconnected pores are stress concentrators and provide a pathway for crack growth, whereas spherical cells can act to blunt the crack tip.
In "Manufacture of a Novel Porous Metal", Int. J. Powder Metall., 1988, vol. 24, no. 1, p. 59, M. W. Kearns et al disclosed that closed cell porosity can be generated in a HIPped P/M material by backfilling a controlled amount of argon, as a pressure-developing medium which exhibits limited solubility in the matrix material, into Ti-6Al-4V powders after canning, with pore formation and growth kinetically controlled in a post-HIP heat treatment for powder consolidation which allows the pressure developing medium to be contained within numerous discrete pores in the matrix material.
In the development of a porous-core, sandwich panel-type structure using Ti-6wt%Al-4wt%V (Ti-6-4) blended elemental (BE) powder, R. L. Martin et al found that introduction of inert gas to metal powder prior to consolidation allows formation of controlled porosity during subsequent heat treatment, causing sufficient diffusion to produce a fully homogenized matrix. Surface densification processing creates an in-situ sandwich structure having a fully dense shell with a porous, low-density core, the gas porosity formed in the metallic matrix being uniform and rounded and therefore behaving innocuously, to produce substantial increases in specific flexural stiffness. Porous Core/BE Ti-6-4 Development for Aerospace Structures, 1991 Powder Metallurgy Conference & Exhibition, Chicago, 1991.
As noted by H. E. Boyer, "Secondary Operations Performed on P/M Parts and Products", "Metals Handbook", Vol. 7, 1984, American Society for Metals, Metals Park, OH, p. 461, porosity also renders some alloys free-machining, so that they require less cutting fluids than their wrought counterparts.
Such results show the potential for reduced density materials in structural applications where weight savings are critical, such as machinery enclosures for acoustic signature reduction, high temperature damping coatings, and energy absorbing barriers.
Porous metallic materials as a group have many unusual properties such as improved acoustic damping properties, improved impact energy absorption, low thermal conductivity, and stability at high temperatures.
Spray deposition offers unique opportunities for the production of composite materials by permitting introduction of phases which would normally be rejected by the melt during ingot metallurgy due to density differences or surface tension effects. During deposition a thick surface layer of the preform is in a semisolid state with equiaxed grains on the order of 50 microns in diameter, as reported by P. Mathur et al, "Process Control, Modeling and Applications of Spray Casting", J. Met., 1989, Vol. 41, no. 10, p. 23.
This type of structure is very similar to that formed during rheocasting and is thixotropic, with apparent viscosity rising sharply with increasing fraction of solid and decreasing shear rate, according to M. C. Flemings, "Behavior of Metal Alloys in the Semisolid State", Metall. Trans. A, 1991, vol. 22, p. 957 and A. R. E. Singe, "A Future for Spray Forming", 1st International Conference on Spray Forming, Swansea, 1990. However, attempts to increase porosity in spray formed materials by deposition under cold conditions result in poor mechanical properties which can be attributed to the highly angular, interconnected porosity that is formed.
There is consequently a need for a method for spray forming a porous metal having closed cell spherical porosity that will impart increased strength to the metal.